A Modeling Tool for Particulate Emissions in GDI Engines with Emphasis on the Injector Zone

Fuel film deposits on combustion chamber walls are understood to be the main source of particle emissions in GDI engines under homogenous charge operation. More precisely, the liquid film that remains on the injector tip after the end of injection is a fuel rich zone that undergoes pyrolysis reactions leading to the formation of poly-aromatic hydrocarbons (PAH) known to be the precursors of soot. The physical phenomena accompanying the fuel film deposit, evaporation, and the chemical reactions associated to the injector film are not yet fully understood and require high fidelity CFD simulations and controlled experimental campaigns in optically accessible engines. To this end, a simplified model based on physical principles is developed in this work, which couples an analytical model for liquid film formation and evaporation on the injector tip with a stochastic particle dynamics model for particle formation. The modeling framework is applicable under steady-state engine operating conditions, and can be extended to transient driving cycle simulations, although the former is only presented in this work. Particle number, mass, and size distributions are validated with experimental measurements performed on a steady-state engine test bench for a 2.0L turbocharged gasoline engine under two engine speeds at part and full load. The model is able to qualitatively capture the main trends in particle number concentration values, especially at the higher load conditions. An underestimation of PN and PM at lower loads is observed, which can be attributed to other sources of particle formation.